1. Field of the Invention
The present invention relates generally to the detection of short circuits in electronic lead frames. More particularly, the present invention provides noninvasive detection of short circuits in lead frames prior to installation of circuits therein.
2. Description of Related Art
In addition to modern integrated circuitry, hybrid circuits make up many of today's sophisticated electronic systems. Hybrid circuits are made from a number of discrete components contained within an integrated circuit package. The hybrid circuit may be in a lead frame which comprises small rectangular wires or "leads" extending outward from a central circuit package. Typically, this circuit package may be generally rectangular or square. The leads extend outward from the package in order to connect the components within the package to other circuits and circuit boards.
The individual conductors, or leads, emanating from the inside of the ceramic or plastic package, which is an electrical insulator, are very fragile and subject to damage in the fabrication process. In order to reduce the vulnerability of the leads to damage, the manufacturer connects all the ends of the outside leads together with a heavy frame of the same material as the leads. The heavy frame similar to a picture frame, maintains the mechanical orientation of the leads during the placement of the components in the package. At the completion of the fabrication process, the "picture frame" is cut off and the circuit is ready for test. Prior to cutting off the "picture frame" the package is technically known as a "lead frame."
Traditionally, the practice of manufacturing hybrid circuit packages has proceeded by loading the package chamber with selected components and covering this chamber after the circuity has been wired. This is a long and costly process. Heretofore, this process has often times been in vain, since any defects in the lead frame such as shorts would be detected only after the initial testing of the completed hybrid circuit. In the event that tests of the fully loaded circuit revealed shorts in the lead frame, the package would have to be discarded, resulting in increased costs for the system.
As is apparent, it would be desirable to test for shorts in the lead frame prior to final assembly of the components into a circuit package. However, this has not been possible because the individual lead frame conductors are shorted at their outside extremity by the lead frame structure.
Thus, a need exists in the art for a method of testing unloaded circuit packages, that is, before the discrete components are assembled to form a functioning circuit. Such a test would allow the hybrid circuit manufacturer to identify and discard defective packages before they are loaded. This type of initial screening would substantially reduce the high cost and lost time associated with the existing practice of testing the circuit package after the final circuit is fabricated.